1. Field of the Invention
The present invention relates to an electric power steering controller and more specifically to a controller for controlling an electric motor to generate steering assist torque.
2. Description of the Related Art
A conventional, electric power steering controller can compensate for inertia of an electric motor or a response delay of a transmission mechanism for transmitting the rotation of an electric motor to a steering shaft by, for example, using a differential output of a torque sensor.
The torque sensor for detecting a rotational force by sensing or detecting a torsion in a torsion bar is generally provided between a steering wheel and the electric motor in a conventional, electric power steering controller. Therefore, when compensating for the inertia of the electric motor by using only the differential output of the torque sensor, the inertia of the steering wheel increases in response to a steering velocity of the steering wheel, thereby causing an adverse influence such as an increase in an overshoot amount of the steering wheel.
In an electric power steering controller described in JP 8-282519A, a target current that should be supplied to the electric motor is determined from the output of the steering torque sensor and a phase compensation value is found from the output of the steering torque sensor and the rotational speed of the electric motor, thus correcting the target current with the phase compensation value. More specially, a value found by phase-compensating for the output of the steering torque sensor with the rotational speed of the electric motor is used to control the electric motor.
As described above, in the conventional, electric power steering controller, the phase delay of rotation of the electric motor to rotation of the steering wheel basically occurs due to torsion of the torsion bar and therefore, the electric motor is controlled so as to compensate for the phase delay.
However, detailed study has revealed that disadvantages are nonetheless present, since the steering assist torque generated by the electric motor is subject to an influence of transmission characteristics in a transmission system that transmits the steering assist torque from the electric motor to right and left front wheel tires. In particular, the steering assist torque is not immediately transmitted from the electric motor to the right and left front wheel tires due to systematic delay. That is, a steering torque transmission system includes a steering shaft, a rack shaft, tie rods and so on. A model of the transmission system includes a predetermined mass and spring elements. Delay in transmission of the steering assist torque is generated by the inertia based upon the mass or spring element or a combination thereof. Accordingly, even if the electric motor generates the steering assist torque in response to a steering input, the right and left front wheel tires are not immediately steered as intended. Thus, the transmission delay can cause an adverse influence on a steering feel and on a driving performance and turning performance of a vehicle.